This invention relates to fuel cells and its used in connection with gas turbines, steam turbines, and heating, ventilation and air conditioning (HVAC) systems, and specifically to high performance hybrid power systems employing such devices.
Conventional high performance gas turbine power systems exist and are known. Prior gas turbine power systems include a compressor, a combustor, and a mechanical turbine, typically connected in-line, e.g., connected along the same axis. In a conventional gas turbine, air enters the compressor and exits at a desirable elevated pressure. This high-pressure air stream enters the combustor, where it reacts with fuel, and is heated to a selected elevated temperature. This heated gas stream then enters the gas turbine and expands adiabatically, thereby performing work. One deficiency of gas turbines of this general type is that the turbine typically operates at relatively low system efficiencies, for example, around 25%, with systems of megawatt capacity.
One prior art method employed to overcome this problem is to employ a recuperator for recovering heat. This recovered heat is typically used to further heat the air stream prior to the stream entering the combustor. Typically, the recuperator improves the system efficiency of the gas turbine upwards to about 30%. A drawback of this solution is that the recuperator is relatively expensive and thus greatly adds to the overall cost of the power system.
Another prior art method employed is to operate the system at a relatively high pressure and a relatively high temperature to thereby increase system efficiency. However, the actual increase in system efficiency has been nominal, while the system is subjected to the costs associated with the high temperature and pressure mechanical components.
Still another prior art method utilized by plants having power capacities above 100 MW is to thermally couple the high temperature exhaust of the turbine with a heat recovery steam generator for a combined gas turbine/steam turbine application. This combined cycle application typically improves the system operating efficiency upwards to about 55%. However, this efficiency is still relatively low.
The overall power system performance is further predicated on the efficiency of the constituent fuel cells and associated cooling systems. The traditional method for fuel cell thermal management is to force high volumes of a cooling medium, either a liquid or gaseous coolant stream, through the fuel cell assembly. Cooling water is often employed for ambient temperature devices, and air can be employed for higher temperature fuel cells. In some instances, the same air which serves as the fuel cell""s oxidant is used as a cooling medium as well. The cooling medium passes through the fuel cell and carries off the thermal energy by its sensible heat capacity. The volume flow of coolant required for this method is inversely related to the limited temperature operating range of the electrochemical operation of the electrolyte, or in the case of fuel cells with ceramic components, by constraints associated with thermal stress.
The foregoing heat capacity limitations on the amount of temperature rise of the cooling medium result in coolant flow rates through the fuel cell much higher than those required by the electrochemical reaction alone. Since these relatively large flow quantities must be preheated to a temperature at or near the operating temperature of the fuel cell and circulated therethrough, a dedicated reactant thermal management subsystem is required. Typically, the coolant is preheated to a temperature either at or near the fuel cell operating temperature, e.g., within 50xc2x0 C. of the operating temperature. Such thermal management subsystems normally include equipment for regenerative heating, pumping, and processing of the excessive coolant flow. These components add substantially to the overall cost of the system.
For illustration purposes, consider a regenerative heat exchanger of a type suitable for preheating the fuel cell reactants and operating with a 100xc2x0 C. temperature difference, and a typical heat transfer rate of 500 Btu/hr-ft2 (0.13 W/cm2). Further assuming a 50% cell efficiency with no excess coolant flow, and operating at an ambient pressure, the heat processing or heat transfer surface area of the regenerator would be of the same order of magnitude as the surface area of the fuel cell electrolyte. Considering an excess coolant flow requirement of 10 times the level required for the fuel cell reactant flow, a representative value for conventional approaches, the heat exchanger surface area would be 10 times larger than the active fuel cell surface area. The large size of this heat exchanger makes it difficult to integrate the heat exchanger with electrochemical converters to form a compact and efficient power system.
Furthermore, the high volume of cooling fluids being passed through the fuel cell makes the fuel cell unsuitable for direct integration with the gas turbine to achieve relatively high system efficiency.
Thus, there exists a need in the art for high performance power systems and for systems that provide for better thermal management approaches, especially for use in electrochemical or hybrid power energy systems. In particular, an improved power system, such as a gas turbine power system, that is capable of integrating and employing the desirable properties of electrochemical converters would represent a major improvement in the industry. More particularly, an integrated electrochemical converter assembly for use with a gas turbine system that reduces the costs associated with providing effective thermal processing approaches while significantly increasing the overall system power efficiency, would also represent a major improvement in the art.
The present invention relates to power systems, and specifically to fuel cell power systems. The efficiency of an operational power system can be assessed by either examining the efficiency of the system of by examining the inefficiency of the system. When examining the inefficiency of the system, the key physical quantity is the energy loss from the system through its gas effluence or exhaust. Typically, fuel cell exhaust contains nitrogen, unreacted oxygen and combustion resultants such as water vapor and carbon dioxide. The energy content released through the exhaust is a function of the exhaust amount. In order to improve system efficiency, the reduction of system inefficiency can be achieved by minimizing the nitrogen content in the exhaust or the air consumption at the reactant inlet. Typically, the high temperature fuel cell system applies excess air (oxidant) for the removal of the exothermic heat release from the fuel cell reaction. The amount of air flow may be as much as five times as high as the stoichiometric requirements. The present invention employs multiple approaches to reduce the air requirements for fuel cell operation.
One approach is to employ a fuel cell that includes an integral lip structure formed on one of the fuel cell plates for heating the reactants as they pass through the fuel cell. The lip structure is described in detail below. The fuel cells employing this lip structure are effective in accommodating reactant temperature rises in excess of 100xc2x0 C. and allowing reduced reactant amounts, thus realizing improved power system efficiency.
The other approach is to employ a fuel cell that includes multiple axially adjacent temperature regions or a collection of fuel cells that operate at different temperatures, for example, in a sequence of increasing temperatures. In this approach, the reactants are introduced into the system at a relatively low temperature and exit at a relatively high temperature. The energy content associated with the temperature change of the reactants is used to cool the fuel cell. In order to maintain a constant power generation of the fuel cell and a fixed quantity of waste heat to be removed by the reactants, fuel cell power systems that operate with larger temperature rises require smaller reactant amounts. When air is utilized as the coolant, the low limit of air consumption for fuel cell operation is known as the stoichiometric rate. Typically, a fuel cell of one electrolyte type provides for only a 100xc2x0 C. rise in temperature of the reactants when passing through the fuel cell. The fuel cell power systems of the present invention is able to accommodate reactant temperature rises in excess of 100xc2x0 C., while employing reduced levels of reactant.
Additionally, the fuel cell power systems of the present invention allow the reactants to be heated under a generally isothermal state locally within the fuel cell. Thermodynamically, the isothermal heating incurs the least amount of entropy, which translates into high system efficiency, such as in the Brayton cycle depicted in FIGS. 5 and 6, in combination with the fuel cell performance. The low temperature fuel cell stack employed in the foregoing temperature cascaded fuel cell design, FIG. 6, has a higher electrochemical potential or a higher fuel cell efficiency than a fuel cell stack operated at a constant high temperature, FIG. 5.
The present invention provides for a system and method for producing electricity with a fuel cell power system. The power system includes an assembly of fuel cell stacks that operate at different temperatures, which vary between two or more of the fuel cell stacks. The system also includes structure for receiving reactants for electrochemically producing electricity. The fuel cell stacks have operating temperatures in the range between about 20xc2x0 C. and about 2000xc2x0 C.
According to one aspect, the fuel cell stacks can be a solid oxide fuel cell, solid state fuel cell, molten carbonate fuel cell, phosphoric acid fuel cell, alkaline fuel cell, or proton exchange membrane fuel cell. Further, the fuel cell stacks comprises a solid state or solid oxide material including yttria stabilized zirconia, a lanthanum gallate, a ceria based oxide, a bismuth based oxides, or a composite of the foregoing materials.
According to another aspect, the fuel cell stack includes a plurality of electrolyte plates having an oxidizer electrode material on one side and a fuel electrode material on the opposing side, and a plurality of interconnector plates for providing electrical contact with the electrolyte plates. The fuel cell stack is assembled by alternately stacking interconnector plates with the electrolyte plate. The fuel cell stacks further can include a plurality of manifolds axially associated with the stack and adapted to receive the reactants. In another aspect, a thermally conductive and integrally formed extended surface or lip of the interconnector plate protrudes into the axial manifolds to heat one or more of the reactants.
According to still another aspect, the fuel cell stack has a cylindrical or rectangular cross-sectional shape, or comprises an array of tubular shaped fuel cells.
According to still another aspect, the exhaust generated by one fuel cell stack is introduced into another fuel cell stack, and a gas-tight enclosure is disposed about one or more of said fuel cell stacks of said assembly. The gas-tight enclosure operates as an outer exhaust manifold for collecting exhaust from the fuel cell stacks. According to one practice, a first fuel cell stack, which generates exhaust at a first operating temperature, is coupled to a second fuel cell stack to receive the exhaust. the second fuel cell stack heats the exhaust to a second operating temperature higher than the first operating temperature. The exhaust can be optionally introduced to the second fuel cell stack as the oxidizer reactant. Furthermore, the second fuel cell stack can be optionally coupled to a third fuel cell stack having a third operating temperature higher than said second operating temperature.
In still another aspect, a number of fuel cell stacks are serially coupled together to heat a fluid, such as the reactants, from a first temperature to a selected temperature. The number of fuel cell stacks are chosen as a function of the selected temperature.
According to another aspect, the power also includes a controller for controlling the amount of fuel supplied to said fuel cell stacks. The controller can include a valve or orifice, or other associated hardware.
According to yet another aspect, the assembly of fuel cell stacks is arranged to form upper fuel cell stacks and lower fuel cell stacks. The upper fuel cell stacks are composed of a material suitable for operation at a first operating temperature, and the lower fuel cell stacks are composed of a material suitable for operation at a second lower operating temperature. A gas-tight enclosure can be disposed about the assembly such that the lower fuel cell stacks are disposed closer to a support structure relative to the upper fuel cell stacks. The operating temperatures of the lower fuel cell stacks can be selected to be different than the operating temperature of the upper fuel cell stacks. Alternatively, the assembly of fuel cell stacks can be arranged to form inner fuel cell stacks and outer fuel cell stacks. The outer fuel cell stacks are composed of a material suitable for operation at a first operating temperature, and the inner fuel cell stacks are composed of a material suitable for operation at a second higher operating temperature.
In another aspect, one or more of the fuel cell stacks comprises multiple axially adjacent temperature regions along the stack, such that each region operates at a different operating temperature. A gas-tight enclosure is disposed about the fuel cell stack for collecting exhaust therefrom. In another aspect, a fluid blocking element is disposed in the fuel cell stack and positioned at a location to selectively occlude one of axially extending manifolds. The blocking element prevents passage of the corresponding reactant within the manifold.
According to another aspect, the fluid blocking element is disposed within said oxidizer manifold, and the fuel cell stack emits exhaust about at least a portion of the periphery of one temperature region, and reintroduces the exhaust to the adjacent temperature region at the periphery and into the oxidizer manifold. The fluid blocking element is disposed at the junction between said temperature regions.
According to still another aspect, the fuel cell stack has first and second adjacent temperature regions. The first temperature regions is formed of a material adapted to operate at a first operational temperature, and the second region is formed of a material adapted to operate at a second operational temperature different than the first operational temperature. The fluid blocking element is disposed at the junction of the first and second regions, and therefore defines the interface between the regions.
According to another aspect, the assembly includes two or more fuel cell stacks forming separate spatially separated fuel cells that operate at different operating temperatures. A gas-tight enclosure is disposed about at least one of the fuel cell stacks and is adapted to collect exhaust from the stack. Structure is provided for coupling the exhaust of one fuel cell stack to another spatially separated fuel cell stack. According to another practice, a first fuel cell stack adapted to generate exhaust at a first operating temperature is coupled to a second fuel cell stack. The second stack receives the exhaust and heats it to a second operating temperature higher than the first operating temperature.
The power system of the present invention can also include one or more compressors associated with one or more fuel cell stacks for compressing one of said reactants, and one or more turbines associated with the fuel cell stacks and adapted to receive exhaust produced thereby. The turbine converts the fuel cell stack exhaust into rotary energy. The system also provides a steam generator associated with the gas turbine and adapted to receive the gas turbine exhaust, the steam generator coupling the exhaust of the gas turbine to a working medium.
The present invention also provides for methods of producing electricity with a fuel cell power system.